Although the invention is not limited to microchip lasers, the invention shall be described in this particular context to demonstrate its particular characteristics.
Microlasers or microchip lasers constitute a new family of lasers having a large number of advantages. A microchip laser is formed of a generally short (ranging typically from 100 .mu.m to several millimeters) amplifier medium framed by two mirrors. This medium is optically pumped by a laser beam generally originating from a laser diode. The power of this diode generally is between about one hundred to several thousands of milliwatts.
The yield of microchip lasers is about between 20 and 30% so that they emit powers of about several tens of milliwatts continuously, indeed several hundreds of milliwatts.
Compared with individual laser diodes, they possess a large number of advantages.
Accordingly, they possess excellent spatial and time coherence converters.
Secondly, owing to the fact that clearly defined energy levels are involved for laser emission and not emission bands as in semiconductive laser diodes, the characteristics of the emission of a solid laser are less dependent on the environment and in particular the temperature than that of a laser diode.
One advantage of this type of laser is their collective method of manufacture. It fact, it is only necessary to coat an amplifier material plate with suitable reflecting films and cut the microchip lasers in their entirety. Since it is possible to start with one plate a few centimeters in diameter and since only a square section of one millimeter suffices to produce a microchip laser, several tens or hundreds of microchip lasers can be embodied in a single cycle of technical operations.
Thus, the production cost of these components is relatively low.
The lasers operate with a continuous pumping beam emitting a continuous luminous beam. However, one advantage of solid lasers pumped by laser diodes is that they use amplifier materials whose radiative lifetime is relatively long (from 100 .mu.m to 100 ms) compared with the lifetimes involved in laser diodes, (which is one nanosecond). It is therefore possible to produce Q-Switch lasers capable of emitting short light pulses (from one fraction of one nanosecond to a few dozen nanoseconds) by storing the energy generated by optical pumping for a period of about the radiative lifetime of the material and by restoring all this energy during a very short period.
This type of short and intense pulse may be obtained with a single laser diode, having regard to the extremely short radiative lifetime periods observed in semiconductors (of a few nanoseconds). These short lifetime periods limit the possible storage period and thus reduce the advantage of these solutions.
Owing to the above-mentioned reasons, microchip lasers increasingly appear as light sources complementary to laser diodes, and are also essential.
Amongst the applications aimed at, the embodiment of microlidars for detecting obstacles seems to about to assume particular significance.
The principles involved may be multiple but they are still based on firstly the time taken by a laser beam to cover the return distance between the emission point, secondly the obstacle which sends back (generally by transmission) one portion of the luminous beam towards the detector, and the thirdly the latter. So as to do this, it is possible to use phase measurements associated with frequency ramps (heterodyne detection) or flight time measurements of an extremely short laser pulse. In the first case, the laser used emits continuously and in the second case, it emits in pulses.
Indeed, there exists many variants concerning these two well-known operating methods.
As regards applications within the motor vehicle industry, for example, this detection of obstacles must, if possible, be carried out, not only in one direction, but in several directions at the same time so as to have a true "picture" of the obstacles situated in front of the vehicle.
This requires the use of a network of microlidars, which requires a complex mounting with mechanical means, which is likely to be subject to deformations owing to the acceleration or extensive movement which may be carried by a vehicle on which this type of system is carried.
Thus, a simpler type of system is needed to avoid the presence of mechanical parts.
Moreover, also required is a system able to incorporate the Q-switching means in a simple form making it possible to retain the compact structure of the microchip laser, avoid as much as possible introducing optical glue and having to carry out optical adjustment during use and avoiding the use of the standard codoping solution unable to adjust the properties of the laser material and a saturable absorbent.
As far as we know, this type of system has not yet been proposed, neither as regards the general context of lasers nor that of the particular context of microchip lasers.